Why would they want to communicate with us?
We still appreciate the art, music, philosophy, stories, etc. of our own low-tech ancestors. So, even if our technology lags theirs, we should still be at least somewhat interesting to an advanced extraterrestrial civilization. Especially if the cost of that information is low.
Thus, all half-dozen of us in our seven light-year neighborhood have a lot of motivation to learn about the others. Given the time lags that even speed-of-light imposes, it makes sense for each of us to stream data continuously to each star system that is streaming information to us. The outgoing information is already a sunk cost to the civilization that developed it; the incoming knowledge is of incalculable value! Some of the others in our neighborhood may have the technology necessary, may have learned the protocols from older civilizations farther out, and may already be in the net.
Third question: if they are motivated to communicate, what rules and protocols would naturally evolve?
To start with, each civilization is highly motivated to develop as many trading partners as it can. It is logical, then, to assume it would use its communication system to "ping" every star in the neighborhood while listening to see if anyone responds with a similar signal. As described further below, these pings would be standardized to require only the lowest cost and technology base to detect, so that, as soon as a civilization reaches the earliest possible capability for two-way interstellar communications, it can also afford to build the necessary systems without certainty of payoff. Rules and protocols would have to be simple and easilyinferred so that new, isolated civilizations, like ours, would be able to figure out how to detect and join their community. The Primary Rule is probably simple: "the more you give; the more you get," but it has some interesting implications, as we will see.
We only need concern ourselves with the simple protocols for the initial ping since directions for more complex ones would be included in the initial data stream. That is, enough protocol information should be implicit in the first ping that we can start a reasonably efficient outgoing data stream without waiting for the 14-year turnaround time. In his novel Contact, Carl Sagan proposed that the first such signal would be a palimpsest of successive amplitude, polarization, frequency, and phase modulations. While a valid concept, his choice of multiplexing protocol places very expensive requirements on the receiving gear. As we noted above, our potential information trading partners in the IITC would want to keep it cheap for us, so that we would be able to join as soon as possible. Therefore, message protocol would be more likely to consist of only a slow sequence of (relatively) long pulses that would be very easy to detect, but have more detailed information contained in intrapulse frequency modulation. The top-level pulse sequence would probably be perfectly regular in repetition frequency and pulse length so that Fourier analysis could pull the weak signal out of a noisy background without a priori knowledge. "Missing" pulses could then encode a very simple message—Sagan suggested prime numbers. Once detected, multiple coherent repetitions of the signal could then be "stacked" to enhance signal-to-noise ratio for decoding the intrapulse message. Sort of a bootstrapping scheme, in which each stage of detection enables the next. If we are lucky, the pings will contain some low bit-rate excerpts from their "big" data stream. A few pictures of themselves and their world would be nice.
Rules for the broader information trading community will almost certainly include the requirement for relay. Each member of the Earth's local group is also a member of its own local group, including us, of course, but perhaps not including one of the civilizations in Earth's group that is on our other side, and thus farther away from them. Recalling the Primary Rule from above—"the more you give; the more you get"—all of us should be anxious to include the relay of data from "farther out" in our own feeds to the members of our local group. In this way, information feeds from civilizations throughout the galaxy could be available to us. Such relayed feeds from far away might be hundreds of thousands of years old.
Another less-apparent consequence of the Primary Rule is that no civilization may be judgmental about the content of the feeds from another. That is, no censorship! Given the likely differences in biology, evolution, and culture; the information from any one culture is certain be repulsive to at least one other race in its multi-hop transmission path. We will be hard pressed to remember that rule and conscientiously forward data from a species that, for example, considers, as an art form, the proper preparation of their young... for consumption.
Fourth: what are the hardware requirements for an entry-level system implied by the above rules and protocols?
As noted previously, the first hardware requirement is that it must be affordable.
Over millions of years of previous interstellar communications, hardware requirements for entry-level systems will have become well-standardized. Thousands of trial-and-error refinements will have produced a first-contact system standard that now has the maximum likelihood of being inferred accurately and quickly by the maximum number of civilizations.
It may seem paradoxical to suggest that two technological civilizations that know nothing about one another, that can not yet communicate, and that are mutually unsure of whether the other even exists, could still somehow agree upon standards. The only way to accomplish such a feat is for each culture to always think about the problem from the other's point of view. Our first lesson from that rule is that since it is the most obvious way to establish standards, it is the actual way in which our opposite numbers will establish standards with us.
So, what do they know about us? They know that we wish to join the IITC and have arrived at its theoretical existence by some logic similar to the one presented here. From their point of view, any other assumptions would be equivalent to ours being just another sterile or primitive solar system, and therefore would warrant no change to their time-tested strategy.
They also know that we, or any rational culture, cannot commit vast amounts of resources to a goal that is no more than a theoretical possibility. (No one ever suggested that the Great Pyramids were a rational undertaking.)
They know that whatever is simplest and cheapest for most of the civilizations in the galaxy is probably simplest and cheapest for us, as well, at this early stage in our technology. They know that we must arrive, eventually, at the same conclusion. Therefore, whenever we actually get around to making contact, we will know that it is the first such system we could have developed that would still be the one that defines the standard for first contact.
So what is the first system capable of interstellar communication we (and therefore others) could develop?
Radio Frequency (RF) communication has been around for a long time and we are very good at it. In fact the earliest proposed standard for incoming interstellar communication was RF. This was first addressed in the initial Search for Extraterrestrial Intelligence (SETI) concept.
The original logic for the choice of frequencies at which to listen was tenuous, at best. SETI traditionally listens to frequencies between 1420 MHz (the hydrogen molecule) and 1662 MHz (the hydroxyl radical). Hewlett Packard Vice President Bernard Oliver wrote in 1971, "Surely the band lying between the resonances of the disassociation products of water is ideally situated and an uncannily poetic place for water-based life to seek its kind." Poetry aside, a more practical reason for the choice was that equipment to monitor these frequencies was readily available within the technology of the time. In his book Interstellar Net: Origins, Edward M. Lerner takes the "Water Hole" logic one step further by suggesting that an extraterrestrial civilization might demonstrate intelligence by broadcasting at a wavelength of 6.7 centimeters. As his protagonist, Dean Matthews, says, "The neutral-hydrogen wavelength divided by pi. That's the carrier signal. That sure sounds like a sign of intelligence to me."
Our lack of success listening at these frequencies would seem to indicate that the choice may have been flawed. Or perhaps the logic was, in fact, also adopted by civilizat
ions at our technological level; and there are many out there all listening... but no one transmitting.
It has been suggested that we don't have to transmit, because advanced ET civilizations would be able to detect our broadcast signals which are now fifty to one hundred light-years out. We have to assume, though, that the laws of physics operate the same everywhere. Energy that radiates into space spreads out—becomes weaker—in accordance with an inverse-square law. That is, the energy collected by an antenna of a given surface area will be proportional to the radiated power in the signal divided by the square of its distance from the source. As an example, consider one of our high power broadcast transmitters that sends out its fifty-thousand Watt signal in all directions. At a range of ten light-years, a receiving antenna would have to be four thousand kilometers in diameter to detect our broadcast. That's not impossible, but it's apt to be a very costly project for a civilization just to detect a signal from a "primitive" culture that is not even specifically trying to communicate.
One of the factors in determining two-way range is the communications bandwidth—a measure of how much information per second is transferred. Bandwidth may be expressed in units of frequency: millions of cycles per second, or MHz. Recalling again the Prime Rule, we will want to stream the most complete data out, so that our trading partners will not feel cheated and limit the data that they send to us. (Being "newbies," we could probably get away with less until our technology catches up.) A TV channel, for example, requires about 6 MHz. Global internet traffic in 2011 averaged about 46 million MHz. It would be nice to get to that rate someday, but it is probably far higher than practical with our entry-level system. Knowing that there is considerable range in the bandwidth choice, our trading partners would leave a lot of flexibility in its selection. As a starting place, let's arbitrarily pick 10 MHz.
What is the very best we could do today to design a two-way, interstellar, RF communications system? If we assemble a system consisting of the highest compatible frequency (10,000 MHz), with the largest steerable antenna on Earth having a surface accuracy capable of handling that frequency (Effelsberg 100-m Radio Telescope in Germany), with the most powerful RF transmitter (two million Watts), and assuming the same system at the other end; the maximum range for link closure is only 1/10 light-year! Scaling the system up with multiple such assemblies and backing way off on bandwidth violates our affordability and functionality criteria for a standardized system long before reaching our seven light-year goal.
Radio frequency systems, therefore, cannot be the entry-level standard our trading partners will be using.
Well, is there any concept that will allow seven light-year communications within our near-term technological capability?
Laser communication, both free space and fiber optic, is now coming into its own, and seems to have the characteristics we are looking for. It has a very high information-carrying capacity, and the smaller wavelengths associated with light work to our advantage. The narrowness with which a beam of light or radio waves may be focused is proportional to the size of the reflector, in wavelengths. That is, for a given wavelength, the bigger the mirror, the narrower the beam; for a given mirror, the smaller the wavelength, the narrower the beam.
Narrowness is good for the outgoing signal; it allows us to focus all the energy we are sending out on a smaller target. That is, it makes our signal look stronger to the ones receiving it. For the incoming signal, narrowness is also good. It allows us to catch more of the energy coming from a smaller region. That makes the incoming signal appear stronger.
For the purpose of allowing our potential trading partners to create a standard for an entry-level communications system, lasers have another very important advantage over radio waves: there are only a finite number of types of laser, and most have only one primary wavelength. In fact, we can assume that the standard will be a laser with a single primary wavelength for that very reason! And that implies that if we find a small number of lasers, preferably only one, that meet interstellar communications requirements with today's technology, we then know exactly where to listen for the incoming ping. And that is also why we need to design a two-way system even though we will just be listening at first. The design of the transmission subsystem will tell us how and where to listen for their transmission.
Let's start with the transmitter this time. Northrop Grumman has demonstrated a continuous-wave solid state laser with a 100 kW output power. The design is based on neodymium-doped yttrium aluminum garnet (Nd:YAG) crystals. It operates in the near infrared at wavelength of 1064 nm (a frequency of 281,946,000 MHz). It is one of the higher powered lasers today.
The Nd: YAG laser is a particularly good choice for communication because its wavelength of 1064 nm gets through the Earth's atmosphere with only about 40% losses— much less if it is based on a mountaintop.
An almost ideal pairing with the Nd:YAG laser for communications purposes is the W. M. Keck two-telescope astronomical observatory. It is near the summit of Mauna Kea in Hawaii, where atmospheric attenuation at 1064 nm is close to zero. It has adaptive optics which means that it is affected very little by atmospheric irregularities, and it is compatible with the Nd:YAG laser wavelength.
Let's do some link calculations. Feeding a single 10 m telescope with a single 100 kW Nd:YAG laser, using a cooled 10 m telescope for receiving, and using the same bandwidth of 10 MHz, provides a communication range of 9.9 light-years! This may be what we are looking for!
The existing two-mirror Keck system can work together as the Keck Interferometer. The 85-meter separation between the two telescopes gives them the effective angular resolution in one direction of an 85-meter mirror. Along this axis, the Keck Interferometer has a spatial resolution of 12 nrad at 1064 nm wavelength. Adding just one more equidistant mirror, and feeding each one with phased Nd:YAG lasers, would provide 300 kW of output power into an 85 m effective aperture. With the same three-mirror set up in the receive mode (cooled), and using a 1 GHz bandwidth, communication range would be 123 light-years. This system will work.
Although this system meets all the theoretical requirements of the standardized entry-level system, we should also consider other bands/apertures to ensure we are not missing the actual "first" system our trading partners are expecting. Looking first at longer wavelengths/lower frequencies:
For sheer power, you can't beat a CO2 laser! And, like Nd:YAG, it is also in an atmospheric window. In a 1989 paper on laser launch systems, Lawrence Livermore National Laboratory researcher Jordin Kare states: "A 20 MW entry-level launch system could be built using current technology at a cost of five hundred million dollars or less." Communication range scales directly with frequency, but only as the square root of power. This means that with one-tenth the frequency (almost exactly) and one hundred times the power, a CO2 laser has the same range as the Nd:YAG laser. Given the power generation and heat rejection problems of such a beast, however, Nd:YAG is still the far better solution.
What about shorter wavelengths/higher frequencies? For the future, this is clearly the direction to go. A space-based far-ultraviolet/x-ray (1—10 nm) laser system would have far greater bandwidths, and longer access times. In fact, such a system may well be the workhorse of our theorized information-trading community. Right now though, instead of the multiple kilowatts at 10 nm that would be required for the outgoing signal, about the best we can do is 5 W at the far-longer wavelength of 266 nm. We are a long way from being able to build such a system.
We therefore conclude that the first system that our IITC trading partners would expect us to build is the system we can build: Two arrays of mountaintop 10-m mirrors, with adaptive optics, set up for interferometry, capable of operating at the Nd:YAG laser wavelength of 1064 nm. For transmitting, each mirror in the transmit array would be fed by a 100 kW Nd:YAG laser modulated in accordance with the first ping we detected.
Our fifth question was: what would we need to do in order to determine the existence of that community?
&nb
sp; Source: http://www.atlasoftheuniverse.com/20lys.html
Clearly, we have arrived at our entry-level system parameters. As we shall see, though, it is neither necessary nor advisable to expand the Keck observatory to these parameters right away. Using just one of the Keck telescopes, uncooled, on the receive side, and backing off on the bandwidth requirement will still allow detection of an initial ping at a range of tens of light-years. Given the above analyses, we were too modest in assuming that our trading partners had to be within seven light-years. We will now increase the range we are considering to twenty light-years.
In order to determine the existence of information trading partners, we will need to look for a Doppler-shifted 1064 nm signal coming from every star within a twenty light-year radius.
There are 83 known star systems within this distance containing 109 stars and eight brown dwarfs. Six of them are Type G stars like our Sun. Many of these star systems are unlikely to have originated life because of high variability or extremely low luminosity. Nevertheless, they might still have worlds, moons, or asteroids that are marginally habitable with the use of advanced technologies. Those may have been "resting places" in the outward expansion of older civilizations described above. They might even be automated relay sites from other nearby systems interested in enhancing their status under the Primary Rule.
Our limitations in observing these star systems are the same as their problems in pinging us: Sun in the way, transmission laser on the wrong side of their planet or moon, insufficient systems to look at more than one star system at a time, etc. Without even knowing the tilt of the plane of their ecliptic with respect to ours, it is difficult to estimate the amount of time any one civilization would be able to spend pinging us. Probably the best we can do is assume that our hypothetical neighbors divide their attention equally among the hundred or so stars around them on a very irregular schedule. As current members of the IITC, they almost certainly will have invested heavily in space sensors capable of directly detecting the Earth and other nearby worlds. Thus, they can focus directly on the worlds they see in neighboring systems. But we are not as well-informed as they are.
Analog Science Fiction and Fact - 2014-03 Page 18
